Severe embrittlement of copper pillar bumps electrodeposited using JGB as leveler

Journal of Materials Science: Materials in Electronics - In this work, the electroplating of copper pillar bumps in wafer used for chip-scale interconnect was investigated. It was found that the...     

The coupling effects of thermal cycling and high current density on Sn58Bi solder joints

Currently, one of the serious challenges in microelectronic devices is the miniaturization trend of packaging. As the decrease of joint dimension, electromigration (EM) and thermomechanical fatigue become critical issues for fine pitch packaging. The independent mechanisms of EM and thermomechanical fatigue are widely investigated and understood. However, the coupling effect of both conditions needs further exploration. The current study established the correlation between resistance and microstructure evolution of solder joint under the combination effect of thermal cycling and high current density and illustrated the different contributions of these two factors to the reliability of the joint through the comparison monopoly tests. The results revealed that cracks had more impact on resistance increase than phase segregation. The resistance evolution could be divided into three stages. First, the resistance mitigated due to the phase coarsening. Second, Joule heating effect made the resistance increase slowly. Third, EM led to the resistance increase rapidly. The high current density can help to improve the reliability of the solder joint under the coupling effect of thermal cycling and EM at the initial stage, but harmful to the consequence process.     

Illustrations of a microdamage model for laminates under oxidizing thermal cycling

Degradations initiated near the edges of a laminate can have a significant effect on its state of degradation, even at the core. Indeed, results from the literature show that laminates which have the same stress state at the core can have completely different states of degradation, even far away from the edges. The paper discusses the influence of the edge effect on damage initiation and propagation for a specific example. A computational micromechanical approach to the degradation of laminated composites was developed recently at LMT-Cachan. This is a hybrid approach in which, depending on the scale, the mechanisms are described using continuous damage mechanics or finite fracture mechanics. Initially developed for static loading, this technique is being extended to fatigue and environmental effects. The aim of this paper is to illustrate the capability of such an approach to take into account major observations during cyclic loading in an oxidizing atmosphere, even when edge effects are significant.     

A strain-energy-density-based lifetime prediction model for notched specimens: Polycarbonate under thermal fatigue

Polycarbonate is more and more extensively used in engineering because of its good mechanical properties. Pieces of polycarbonate are used in environments with variable temperature, especially in electronic devices. Thermal stresses could become important and, for this reason, the effects of thermal stresses must be taken into account in designing these pieces. We propose a method for the prediction of the life of notched specimens based on the density of dissipated strain energy. The laws of behavior of polycarbonate at various temperatures are determined, and the fatigue tests performed on smooth specimens give the laws of thermal fatigue of the material. The fatigue tests on notched specimens and finite-element-method computations enable us to establish the relationship between the stress concentration factor, the density of strain energy dissipated at the notch roots, and the density of nominal strain energy. A life-prediction model is proposed and discussed. Laboratory of Mechanical Reliability, Metz University, Metz, France. Published in Problemy Prochnosti, No. 6, pp. 32–42, November–December, 1998.     

A general model for age acceleration during thermal cycling

Conventional practice in evaluating the effect of elevated temperature upon electronic devices in either accelerated testing or stress screening is the use of the Arrhenius reaction rate equation. This equation fails to represent several key aspects of the stress. A general model that includes the reaction rate effects during heating and cooling, the mechanical effects of heating and cooling, and non-constant activation energies is provided here. The model is based upon a general representation of a thermal cycle. Once the general model is constructed, it is shown to accommodate as special cases, acceleration when one or more of the features is assumed absent. An example illustrates this point. Then several uses for the model beyond the computation of the acceleration factor are discussed. It is suggested that the general model can be used to support test design and equipment selection decisions. The model therefore provides a more realistic portrayal of the effects of a thermal cycle and increased decision flexibility in defining thermal stress regimens.     

Effect of thermal cycling process parameters on recrystallization kinetics for processing of fine-grained pure copper

The present work developed a two-step thermal cycling technique for processing of fine-grained pure copper for improved properties. This included initial annealing of specimens followed by heavy cold rolling and a series of heating and cooling cycles. The study investigated the important microstructural changes occurring in the cold deformed grains in the absence of phase transformations. A major interest of the present research was to closely describe the effect of thermal cycling parameters on the recrystallization and grain growth kinetics for processing of fine-grained structure. The study determined the optimum values of process parameters for the developed thermal cycling route including the extent of cold deformation, annealing temperatures, holding periods, and the number of thermal cycles. The thermal cycling process produced closely distributed fine grains with bi-modal microstructure leading to increased hardness and strength without any significant loss in electrical conductivity.     

Thermodynamic constitutive model for load-biased thermal cycling test of shape memory alloy

This paper presents a three-dimensional calculation model for martensitic phase transformation of shape memory alloy. Constitutive model based on thermodynamic theory was provided. The average behavior was accounted for by considering the volume fraction of each martensitic variant in the material. Evolution of the volume fraction of each variant was determined by a rate-dependent kinetic equation. We assumed that nucleation rate is faster for the self-accommodation than for the stress-induced variants. Three-dimensional finite element analysis was conducted and the results were compared with the experimental data of Ti–44.5Ni–5Cu–0.5 V (at.%) alloy under bias loading.     

Low-temperature thermal expansion of copper: Search for specimen-dependent effects

Five samples of copper have been intercompared in an optical lever dilatometer to ascertain if specimen-dependent effects exist in the thermal expansion of nominally pure copper at low temperatures (T<10 K). The intercomparison has been made using the static piezoelectric effect of a quartz crystal as a transfer standard. Efforts have been made to keep the internal systematic error to a minimum during the comparisons. The absolute systematic errors have been estimated by including a sample of the N.B.S. copper standard and a specimen previously measured by the University of Iowa group in the comparisons. The maximum disagreement with the latter is 10% nearT=6 K The internal error is expected to be much smaller than this. The results show that no two of the present specimens are in agreement within several times the scatter of the measurements, ± 2%. The largest change, 20%, was caused by annealing of very pure copper in an oxidizing atmosphere. The correlation of the results with residual resistivity ratio measurements and other factors suggests that an impurity-dislocation interaction may be the cause of anomalous expansion.This work was supported by the National Research Council of Canada.     

Positron lifetime spectroscopy characterization of thermal history effects on polycarbonate

The effect of a short-term anneal aboveT _g on the free volume cavity size and concentration and on the fracture toughness of polycarbonate is examined. The positron annihilation lifetime (PAL) technique is used to measure the change in free volume concentration and cavity size during isothermal relaxation experiments at 10, 20 and 30 ° C. An activation energy of 16.5 kJ mol^–1 is calculated for the relaxation of the annealed polycarbonate, compared to 12.3 kJ mol^–1 for the unannealed material. The fracture toughness and brittle fracture morphology of compact tension specimens are unchanged by the anneal. The similarity in the PAL parameters and physical properties between the unannealed polycarbonate and the material annealed aboveT _g suggests that the short-term anneal does not appreciably alter the structural state of glassy polycarbonate.     

Mathematical model of the process of thermal destruction of polymer materials under intense thermal effects

It is proposed to describe heat- and mass-transfer processes in decomposing materials by taking into account both the chemical and physical transformations and the phase transition temperatures.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 53, No. 5, pp. 774–781, November, 1987.     

Reliability of lead-free solder joints in CSP device under thermal cycling

Finite element method and Garofalo–Arrheninus creep model were combined and used to evaluate the reliability of different lead-free solder joints (SnAgCu, SnAg, SnSb and SnZn) and SnPb solder joints in chip scale package (CSP) 14 × 14 device under thermal cyclic loading. The results show that von Mises stress and equivalent creep strain in each of the four lead-free solder joints and SnPb solder joints were strongly different, increasing in the order SnPb < SnAg < SnSb < SnZn < SnAgCu. It is found that maximum stress–strain concentrates on the top-surface of corner solder joints in the CSP device for all solder joints, and SnAgCu solder joints shows the highest fatigue life among those five kinds of solder joints.     

Prediction of fatigue lifetime under multiaxial cyclic loading using finite element analysis

Life prediction for GH4169 superalloy thin tubular and notched specimens were investigated under proportional and nonproportional loading with elastic–plastic finite element analysis (FEA). A strain-controlled tension–torsion loading was carried out by applying the axial and circular displacements on one end of the specimen in the cylindrical coordinate system. Uniaxial cyclic stress–strain data at high temperature were used to describe the multi-linear kinematic hardening of the material. The comparison between FEA and experimental results for thin tubular specimen showed that the built model of FE is reliable. A fatigue damage parameter was proposed to predict the fatigue crack initiation life for notched specimen. The results showed that a good agreement was achieved with experimental data.     

冷热枪状态下弹枪相互作用的热力耦合分析

为了研究在冷热枪状态下弹枪的相互作用,以某型自动步枪的铜被甲弹丸与枪管为研究对象,建立了其相互作用的热力耦合有限元模型,模型结合了考虑温度对摩擦因数影响的摩擦力子程序以及基于火药燃烧过程的内弹道载荷子程序,对弹丸温度变化及其在全枪管中的运动进行了数值仿真分析。结果表明:冷枪时,铜被甲弹丸表层升温在200℃左右;400℃及700℃的假设热枪状态下,刚完成挤进时铜被甲弹丸表层温度已经接近熔点,在膛内运动过程中表层铜材料剥落,枪管内膛会出现了"挂铜"现象;另外,热枪状态下弹丸在膛内的攻角变化明显大于冷枪状态下的攻角,不仅印证了热枪射击精度比冷枪射击精度差,也揭示了造成此现象的机理:由于高温下铜被甲圆柱部材料软化脱落,变形不规则,造成弹丸在膛内及出膛摆动增大。     

Effect of thermal cycling on the microstructure of continuous carbon fibre reinforced copper matrix composites

Copper reinforced by continuous carbon fibres is a candidate material for heat sinks of electronic modules. The thermal expansion can be matched to the adjacent ceramics and the thermal conductivity is higher than that of alternative materials. Because the material is expected to work in cyclic thermal conditions during application its structure must withstand such a load without any severe damage. The structure of unidirectionally and cross-ply fibre reinforced samples was observed before and after thermal cycling by optical microscopy and Scanning Electron Microscopy (SEM). SEM was used for examining the fibre-matrix interface before and after thermal cycling. Optical microscopy showed the arrangement of individual monolayers. The influence of adding copper foils between individual monolayers on the structure of the material is reflected in a reduction in the number of cracks in monolayers.